A porous honeycomb structure has been broadly used as a filter (diesel particulate filter (DPF)) for capturing/removing particulate matters included a soot-containing fluid such as a diesel engine exhaust gas, or a catalyst carrier for carrying catalyst components to purify toxic substances in the exhaust gas. The honeycomb structure comprises: cell partition walls (ribs) forming a composite body of a plurality of adjacent cells; and a honeycomb outer wall which surrounds and holds outermost peripheral cells positioned in an outermost periphery of this cell composite body. A silicon carbide porous body using fire-resistant silicon carbide particles as aggregates has been used as a material constituting the structure.
As the honeycomb structure, for example, a honeycomb-structure porous silicon carbide catalyst carrier has been described which is obtained using impurity-containing silicon carbide having a predetermined specific surface area as a starting material. The material is formed into a desired shape, dried, and thereafter fired in a temperature range of 1600 to 2200° C. (see, e.g., JP-A-6-182228).
In a sintered configuration (necking) by recrystallization reaction of silicon carbide particles themselves in the catalyst carrier described in JP-A-6-182228, silicon carbide components evaporate from the surfaces of the silicon carbide particles, and condense on each contact portion (neck portion) between the particles, and accordingly the bonded state is attained due to the grow of the neck portions. However, since very high firing temperature is required for evaporating silicon carbide, cost increase is brought. Moreover, since a material having a high coefficient of thermal expansion has to be fired at high temperature, there has been a disadvantage that the yield after firing drops.
Moreover, to produce a filter having a high porosity, especially a filter having a porosity of 50% or more by the above-described sintering by the recrystallization reaction of the silicon carbide particles themselves, a sintering mechanism does not sufficiently function, therefore the growths of the neck portions are inhibited, and accordingly there has been a disadvantage that strength of the filter drops.
As a conventional technique for solving these problems, there have been disclosed a porous honeycomb structure containing fire-resistant particles which are aggregates, especially silicon carbide and metallic silicon, and a process for producing the structure (see, e.g., JP-A-2002-201082). According to the producing process, a porous honeycomb structure can be inexpensively produced at a comparatively low firing temperature, and the obtained porous honeycomb structure has characteristics of being comparatively high in porosity, and high in thermal conductivity and strength.
At present, as to the above-described DPF, it is one of important problems to reduce as much as possible pressure losses which largely influence engine outputs. To attain this theme, it is required to have a DPF of increased porosity; that is, a material having a higher porosity be used as a porous material constituting the filter. There is under development a DPF being provided with a regeneration system in which an oxidation catalyst is loaded on a conventional DPF (DPF for catalyst regeneration), and can oxidize to burn deposited particulates for continuously regeneration.
Especially, as to the DPF for regeneration the catalyst, it is required to reduce the pressure loss of the filter as much as possible, and also required to make the porosity higher, concretely 50% or more, and especially around 70%.
However, when the porosity of the silicon carbide porous body is raised, the strength accordingly drops. At a porosity of a certain or more degree, for example, at a porosity of 60% or more, for example, there has been a problem that it is difficult to preferably use the porous body as a material constituting a filter, a catalyst carrier or the like for automobile exhaust gas purification.